The present invention relates to a process for protection of the heat transfer
tubes from overheating / damaging in a Gradating Rukttsed Bed hotter (CFB
Boiler) during sudden trip out of power during hotter operation.
A circulating fluidtod bid better (CFB) system consists of a combustor, solids
recycle loop namely a plurality of cyclone, pressure seal - loopseal, and the Fluid
Bed Heat Exchanger (FBHE).
Fuel combustion In the circulating fluid bed system takes place in a vertical
combustion chamber. Properly sized fuel is fed into the system and is burnt at
relatively low temperature (less than S®0 * C).
The bed material is flukteed by preheating primary air introduced through an
arrangement of nozzle disposed on a grate located at the bottom of trie bed. The
flue gas generated during combustion, flows upwards with a relatively high
velocity to fill the entire combustor with suspended solids. They have a high
concentration dose to the ftukfcing grate but decrease continuously towards the

top of the combustor. The combustion gas then circulates a considerable portion
of the sottds within the combustor and then cany them over to the recycle
cydone where the entrained solids are separated from the gas.
As the soHds are continuously returned to the bed by the recyde loop, this
system is known as circulating fiufcfeed bed. A very high internal and external
circulating results of solids, characteristics of the circulating fluid bed, results in
consistentty uniform temperatures throughout die combustor and the solids
recycle system. Due to this high circulation rates, a unWorm temperature profile
exists in die combustion system preventing temperature peaks. The temperature
can be adjusted by employing ash circulating throiicjh external flukJized bed heat
exchanger wherein the ash is cooled hf means of heat transferring tubes located
in the ash stream. This positive and superlative mechanism thus ensures
operating temperatures in the combustion far below the softening points of die
fuel thus prevent sticking of die ash.
Because of the high slip velocity between gas and solids, die solids proceed
through die combustion at a much lower velocity than the gas. Solids residence
times in the order of minutes are obtained for each cycle of solids circuJation.

The long residence by virtue of redrculation and contact times, coupled with the
smaN particle sizes results In efficient heat and mass transfer rates, provides a
high combustion efficiency.
Combustion air is introduced into the combustor at two levels. A
substochtometrtc condition is ensured at bottom of combustor by admitting only
a part of combustion air as primary fiukfaing air through the grate, and the
balance is admitted as secondary air through multiple ports in the wafts of the
combustor. The combustion proceeds in two zones: a primary reducing zone in
the tower section of the combustor, and complete combustion using excess aft* in
the upper section . This staged combustion, at controlled tow temperatures,
effectively suppresses NOx formation.
Secondary air enters the combustor via multiple ports located in the walls of the
combustor, as fluWizing air for the fluid bed heat exchanger, and siphon seal.
Various seal and fiuWizing air flows contribute to the total combustion air.
The flow of air is automatically proportioned to the fuel feed rate in order to
maintain the required excess air ratio for combustion.

Flue gas resulting from the combustion of the fuel, and the entrained solids, exit
the combustor at combustion temperature in the upper poitim of the coffthustor
water wall and is dueled Into the recycling cyclone ctesigned to remove over 99%
of the solids entrained by the gas from the combustion chamber. For optimum
separation efficiency, the cyclone is equipped with vortex finder. The solids
separated by the recycling cydone is collected in the air flukJized seal pot
provided with solids extraction varves (Ash Control Devices or Spiess Valves).
These valves are used to control the combustor temperature by allowing the
solids into fluid bad heat exchanger.
The soHds passing through the fluid bed heat exchanger transfer a portion
generated of sensible heat to the heat exchanger surface provided in the fluid
bed heat exchanger. The solids are discharged from the heat exchanger and
returned to the combustor via water wad openings.
The fhiidized bad heat exchanger (FBHE) is a key element to the CFB system. It
provides heat transfer surfaces external to the combustor and permits the

extraction of heat from a controlled solids circulation loop while maintaining
optimum combustor performance conditions for a wide range of fuel type and/or
load. For the required heat transfer, portion of the drojlating solkls is ofecnarged
from the cyclone seal pot via the spiess valve to the FBHE.
Since the circulating flukfeed bed belter of prior art as described herein above
contain significant amount of refractory and circulating ash inventory, a
significant risk to the heat transfer lubes located in the vicinity of the healed
mass of refractory Is Nivotved in event of non supply of adequate cooling media
(feedwater), for example, due to total electrical power failure.
It is therefore an object of the invention to propose a process for protection of
the heat transfer tubes (torn overheating / damaging in Circulating Fluidised Bed
Boilers (CFB BoUer) during sudden trip out of power during boiler operation,
which eliminates the disadvantages of prior art.

In case of a station Wackout, the boiler trips and the supply of combustion air is
stopped immediately as well as the bailer feed water supply. Ail relevant control
instruments and actuators are linked to emergency power or Uninterrupted
power supply (UPS) system. The distributed control system (DCS) is operated
using the UPS power.
According to the invention, the cooling down of heat transfer tubes is ensured
by the process steps as described herein below In the distributed control system
of the powerptent. The process is configured from the inventive concept, that
the prior art procedure vents the steam generated during this period to avoid
over pressuring the steam generator. This is followed with emergency feed water
pump brought into operation to maintain the loss of cooling media in the system.
In contrast, thereto the invention circumvent the constraint of continuation of
supply of the cooling media (boHer water) while maintaining the system
pressure within the designed pressure of the safety valve. This is achieved by
calibrating utilization of water inventory in the system in combination

with regulation of system pressure corresponding to the calibrated utilization of
water inventory. The calibrated data is advantageously located in the
uninterrupted power supply connected to an electrical driven cooling fan. The
innovated control means thus utitaes the inherent physical property of the
cooling media (water), i.e. the higher latent heat at lower pressure (calibrated
for the particular system) in extracting heat out of the combustor and avoid
potential damage to the heat transfer tubes. The coottng fan operated in
conjunction with the above emergency activated control means thus protects the
CFB steam generator from the damages due to stored heat content. The cooling
down of the refractory is then safely controlled by utiftting the strategically
located cooling air fans driven by the power supply from UPS.
Rg. 1 - shows a typical CFBC holer.
Rg. 2 - shows a process Mow chart depicting the prior art process vis-a-vis the
inventive process.

The control means of the Invention cods down the steam generator to protect
the boHer tubes Moe, combustor water waits, SH and RH tube bundles (which are
immersed in hot ash In FBHfe) under better trip conditions.
Rg. 1 shows a schematic of a typical CFBC boiler. It is essentia) to protect the
CFBC bolter under boiler trip / station black out conditions. In CFBC boilers,
enormous residual heat are avatobte in the refractory watts of the cyclone, in the
combustor and in the hot bed materials / ash. This residual heat is transferred
through radiation and conduction, and accordingly generation of steam is
ensured for certain time even after hotter trips.
Hence unlike conventional boilers, it is not necessary to maintain sufficient water
flow through the holer tube for certain time whenever the boHer trips, to protect
the boiler tubes. SimUarty, the spiess valves, which are in direct contact with the
hot ash to control the hot ash flow, are neither needed to be cooled continuously
with cooling water even after boiler trip. According to the prior art, due to the

rapidly decreasing release of heat from the combustors refractory and due to
the self insulation effect of the ash inventory, in case of non operation of the
emergency boiler (lead water pump, the evaporator stops after the water level
has fallen below the area in the upper part of the combustor, which is exposed
to long lasting radiation heat from the cyclones. This phenomenon results hi that
cooling of the area by evaporated water/steam would no longer take place
leading to a cleforrnation of the water wall including erosion of a preferred
location immediately after start-up. Hence this condition is to be avoided.
In order to be able to operate and protect the boUer and its components property
during station black out the following arrangements are provided.
The stored latent heat in the ash inventory, refractory in the lower part of the
combustor and the refractory lining in the cyclones is continuously released to
the evaporator surfaces of tt»e combustor by the foliowing manner:
• Direct heat transfer from the refractory lining of the combustor's
lower part to the evaporator membrane wall covered by the
refractory.

• Heat radiation from the surface of the ash inventory, which has
fallen down to the bottom of the combustor, to the evaporator
walls of tt» combustors upper part.
• Heat radiation from the refractory Hiiing of the cyclones through
the inlet openings hack to the evaporator water walls of the
combustor.
• Natural convection to the evaporator walls.
• The ash inventory which fell down on to the bottom of the
combustor after shut down starts to insulate iteeff after having
settled. For the later, restart of the hotter it is important to know
that after starring the fluidizatjon, the bed's latent heat stored in
the ash inventory at the bottom of the combustor is immediately
exported to the evaporator walls of the combustor.
As a result of the above measures, there wMI be intense
evaporation and superheating in FBHEs and a back pass in a first
phase of a station blackout. Later, the steam flow is continuously
decreasing due to loss of talent heat in refractory, ash inventory
£»nd due to the self insulation effect of the ash.

The water-steam-mixture of the evaporator gets separated by the different
densities of water and steam due to the (basic) steam parameters of the boiler
namely, pressure and circulation ratio (steam content) of the risers. As long as
evaporation continues in the water wets of the combustor, sufficient cooling of
all boNer parts, which are exposed to heat radiation and convection, is ensured.
The water tosses is compensated by feeding feed water with an emergency
boiler feed water pump to the drum via an economizer by maintaining a
required drum level. The emergency boiler feed water pump remains operative
latest after 1 minute.
The preheated boiler feed water te extracted from the deaerator. Due to the
limtted size of the deaerator, cold boiler feed water is fed to the deaerator with
the boiler make-up water pump, connected to the EPS. Cold boiler feed water is
not fed directly to the drum via economizer but via the boiler filing line
connected to the combustor watt inlet headers to avoid a thermal shock and
strain for the drum. These cooling are achieved conventionally in a CFS boiler by
not tripping theBFW pump when the boiler trips, and through SG cool down

logic by controlling steam / water flaw through various drains and vents. If the
boiler trips due to shortage of feed water Hke a low feed water tank level, a low
drum level, BFWP frips or due to station black, out then the boiler is cooled wHh
the water available in the emergency tank by running the emergency boHer feed
pump (EBFWP).
The 96 cool down logic is actuated when tiie boiler trips on BP-I actuated when
combustor lower temperature is more than the prescribed value greater then
specified temperature value in the typical response in case of SG cool down
actuate the following action wW be initiated to achieve the cooling of the boHer
tubes.
1) Drum level control forced to auto to maintain the drum level.
2) FW low load control valve isolation valve open.
3) Established FW low load control valve in auto.
4) 100% control valve isolation valve to be closed manually.
5) Steam vented out through superheater / reheater startup vent.

6) Cksse main steam Hne and strainer drain valves.
7) Close HP bypass drain valves.
8) Feedwater flow low alarm shall be generated if the flow is